Assessment of the bioavailability, bioaccessibility and transfer of heavy metals in the soil-grain-human systems near a mining and smelting area in NW China

Abstract

Elucidating the transfer behaviour of heavy metals from soils to grains and ultimately to humans is of great significance for both human health risk assessment and pollution control. In this study, the bioavailability of heavy metals (Cd, Cu, Pb, Zn, Cr and Ni) in farmland soils and bioaccessibility in grains (spring wheat, maize and rice) were determined to elaborate transfer dynamics in the soil-grain-human systems near a mining and smelting area in the Dongdagou watershed, Baiyin district, Gansu province, NW China. The results showed that Cd, Cu, Pb and Zn concentrations in soils were elevated compared to background levels, while Cr and Ni concentrations were relatively low throughout the region. High levels of bioavailable soil Cd were found using both EDTA and CH₃COOH extraction methods. Mean concentrations of Cd, Pb and Zn in spring wheat grains and the Cd and Cr concentrations in maize grains exceeded the relevant maximum levels for pollutants according to the Chinese national standards for food safety. Except for Ni (41.90%) and Pb (31.39%), heavy metal bioaccessibility was relatively low in grains, ranging from 10.80% (Cd) to 17.18% (Zn). CH₃COOH-extracted Cd, Pb and Ni, EDTA-extracted Zn, and total Cu in soils were the best indices for evaluation of uptake in grains (R²=0.54-0.91, p<0.001). Internal exposure doses of Cd and Ni in humans from spring wheat grain consumption was predicted by the linear correlations between bioaccessible and total metal concentrations (R²=0.61 and 0.67; p<0.001). The results from this study provide sufficient data and theoretical support for the use of these methods for local pollution prevention and control.

Keywords: Agroecosystem; Bioaccessibility; Bioavailability; Heavy metals; In vitro stimulated digestion; Transfer behaviour.